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ESTRO 35 2016 S397

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of automatic (AU) and manual (MA) generated H&N VMAT

plans created for clinical use.

Material and Methods:

All patients (n=30) referred to

curative H&N radiotherapy in August and September 2015

were planned with a MA and AU VMAT plan in Pinnacle

version 9.10. Half of the tumours were located in the pharynx

(15) and the rest were mixed between larynx (4), oral cavity

(3), salivary glands (2), thyroid gland and unknown primary

(4). The plans followed national guidelines, and planning

techniques were blinded before clinical evaluation of senior

oncologists. The MA plans were optimized according to

standard clinical practice. The AU plans were created by the

Autoplan software available in the Pinnacle planning system.

After AU optimisation, slight manual fine-tuning of the plans

was performed. To supplement the clinical evaluation the

operator time for the dosimetrist was recorded along with

DVH parameters and the global detector pass rate (3% and

3mm) of the delivered plans on an ArcCheck phantom. All

statistical analyses were performed with a paired Wilcoxon-

signed rank test.

Results:

In 29/30 plans, the AU plan was chosen for clinical

application (p<0.001). In terms of DVH parameters similar

target coverage was obtained between the two planning

methods. As seen in the table, mean OAR doses were

significantly reduced in the AU plans for all organs. The mean

reduction ranged from 0.5 Gy for the entire patient to 6.5 Gy

for the contralateral submandibular gland. Differences in DVH

showed significant AU superiority in the dose range 10 Gy to

45 Gy for all organs (Mean DVH example shown in figure). The

only manual plan selected for clinical use was a thyroid

cancer plan involving level VII lymph nodes and therefore

included a large volume of the lung, which had a lower lung

dose in the manual plan. The AU plans were more modulated

as illustrated by the increase in MUs, which might cause the

reduced, but still clinically acceptable, pass rate of 97.7% in

ArcCheck measurements. The increased beam-on time of 4

sec is clinically unimportant. Mean operator time spent on MA

plans was more than twice that of AU plans. The target

homogeneity, conformity and dose fall off were all superior

in the AU plans.

Conclusion:

All AU and MA plans were of acceptable clinical

quality, however, AU plans resulted in reduced doses to all

OAR and required less operator time in the planning process.

AU plans were almost consistently preferred by senior head

and neck cancer specialists. The dosimetric superiority of the

AU plans was evident.

PO-0838

Impact of dosimetric outliers on the performance of a

knowledge-based planning system

A. Delaney

1

VUMC, Radiotherapy, Amsterdam, The Netherlands

1

, J. Tol

1

, M. Dahele

1

, J. Cuijpers

1

, B. Slotman

1

, W.

Verbakel

1

Purpose or Objective:

RapidPlan (Varian Medical Systems) is

a knowledge-based planning solution based on a model

derived from a library of previous treatment plans. The

model utilizes the geometric features and associated

dosimetry of these plans to predict a range of achievable

dose volume histograms (DVHs) for each organ at risk (OAR)

of a new patient. RapidPlan (RP) drives the VMAT or IMRT

optimization process by placing a line of optimization

objectives along the lower boundary of the DVH prediction

range. Planning inconsistencies may lead to sub-optimal plans

in the model, which can be identified as dosimetric outliers.

Outlier cleaning is advised, however this is time consuming

and often subjective. We examined the effect of model

cleaning and increasing numbers of dosimetric outliers in a

model library, on RP plan quality.

Material and Methods:

70 head and neck cancer treatment

plans (planned consistently using the same departmental

objective priorities) were used to populate uncleaned

ModelUC. Statistical metrics provided by RP were used to

identify geometric/dosimetric outliers in ModelUC which

were then visually assessed and, if appropriate, removed to

create cleaned ModelC. The last 5-40 patients (increments of

5) of ModelC were then re-planned with no attempt to spare

the salivary glands, and afterward re-introduced to ModelC,

creating Model5-40. All models were used to create plans for

a 10 patient evaluation group. Although RP can generate OAR

objective priorities, for this study, the same standard

priorities were used as for the plans in ModelUC. Plans were

compared on the basis of generated prediction ranges,

boost/elective target volume homogeneity index (HIB/HIE),

mean dose to the oral cavity (OC) and to composite

structures comprising the salivary glands (compsal) and

swallowing structures (compswal).